Precipitation of monohydrate alumina in the Bayer process

Project Information

Grant agreement ID: JOE3950003

Project closed

Start date 1 January 1996

End date 30 June 1997

Funded under

• Specific programme for research and technological development, including demonstration in the field of non-nuclear energy, 1994-1998

Total cost

No data

EU contribution

No data

Coordinated by

NATIONAL TECHNICAL UNIVERSITY OF ATHENS
Greece

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Deliverables

A new process for the production of monohydrate alumina (boehmite) from supersaturated sodium aluminate solutions at atmospheric conditions

The production of alumina by the Bayer process involves three main steps: digestion of bauxite, precipitation of aluminium trihydrate (gibbsite, Al(2)O(3).3H(2)O) from the resulting solution and calcining of the gibbsite at 980-1200 degrees Celsius to produce alumina. The calcination step absorbs 3GJ/t alumina or 25% of the total Bayer process. In the current project a highly innovative modification of the Bayer process has been developed, whereby aluminium monohydrate (boehmite, Al(2)O(3).H(2)O) rather than the trihydrate is the product of the precipitation stage. In this process boehmite is precipitated under conditions similar to these used for gibbsite precipitation (atmospheric conditions at temperatures lower than 100 degrees Celsius). This results in significant energy savings, estimated to 1.8 GJ/t alumina or 60% over the current practice at the calcination stage because (a) the enthalpy of dehydration of boehmite to alumina is lower by 1.1 GJ/t alumina compared to that of gibbsite, and (b) the quantity of material to be calcined will be reduced by 20% per t alumina, as only one rather than three moles of water are associated to one mole of alumina. Because of this huge energy savings potential, it is expected that the proposed Bayer process modification will be applied to all new smelters; it is also possible that the incentive will be high enough so that it will be adopted by existing smelters as well, with the appropriate modifications in the precipitation section. The exploitable result of this project is a new process in which boehmite is precipitated, isothermally or not isothermally, from sodium aluminate solutions at atmospheric conditions and at temperatures lower than 100 degrees Celsius. The project is addressing the development of an energy efficient process (B07) and generally to Process engineering (E33). This process is a highly innovative variation of the Bayer process and has not been applied before in the alumina industry or in any other relevant industries. It comprises a modification of the precipitation section of the Bayer process whereby aluminum monohydrate (boehmite, Al(2)O(3).H(2)O) rather than trihydrate (gibbsite, Al(2)O(3).3H(2)O) is precipitated. This will result in energy savings of 1.8 GJ/t alumina; for an average size smelter producing 1.000.000 t alumina/year, this represents savings of 1.8x10(-6) GJ/year.

Exploitable results

Precipitation of monohydrate alumina in the Bayer process

The objective of the current project was the development of a highly innovative modification of the Bayer process, whereby aluminium monohydrate (boehmite, Al2O3.H2O) rather than the trihydrate is the product of the precipitation stage. This modification will result in significant energy savings during the calcination stage because (a) the enthalpy of dehydration of boehmite to alumina is lower by 1.1 GJ/t alumina compared to that of gibbsite, and (b) the quantity of material to be calcined will be reduced by 20% per t alumina, as only one rather than three moles of water are associated to one mole of alumina. The energy saving in calcination of boehmite is estimated to be 34.8 kg/t of alumina for fuel oil and 7.2 kWh/t of alumina for electricity. This corresponds to 40-50% less fuel or electricity consumption compared with gibbsite calcination for the same alumina production. For a typical Bayer process plant with a capacity of 1.000.000 tpa alumina, this represents energy savings of 35000 t of fuel oil per year. In the present project the condition of boehmite precipitation from aluminate liquors have been studied in detail. A theoretical model predicted boehmite solubility in caustic solutions has been developed and also validated by the experimental data of boehmite solubility in such solutions. Detail kinetic analysis of experimental data has been carried out and a kinetic equation has been established. This equation has been incorporated into a new simulator and used to develop the optimum processing flow sheet for the precipitation section. Finally, a technological and economical evaluation of the new process has been carried out to. The main results of the research performed are the following: - Boehmite is precipitated from supersaturated sodium aluminate solutions at atmospheric conditions in the presence of boehmite seed. - The % yield of the new boehmite precipitation process is very close to this achieved in the current gibbsite precipitation process. - Precipitated boehmite is characterized as "floury" type.